4. Be acquainted with the characteristics of explosives that determine suitability
for military use.

5. Be able to compute oxygen balance and understand its significance.

6. Be able to compute the potential and relative strength of an explosive.

Introduction

To this point, our concentration has been on contact detection (and avoiding it), the sine
qua non for any weapons system. The other essential requirement is a destructive
capability. Research for the Strategic Defense Initiative (SDI) concluded that this
requirement could be fulfilled in the form of "hard kill" (physical destruction of the
warhead) or "soft kill" (mission impairment) using numerous "kill mechanisms" (directed
energy, kinetic energy, ECM, etc.)

Figure 12-1. SDI Kill Levels and Modes

Further study of the "kill" phenomenon requires a dis-
tinction between immediate (within milliseconds) or delayed
kill. The difference results in tracking assets wasted and
the unnecessary dilution of defensive systems in a time
critical situation.

To meet the kill requirements, weapons systems using
three different energy sources have been proposed. Histor-ically, chemical and nuclear explosives have been used.
They, together, form the "Potential Energy" weapons (PEW)
group. More recently, Kinetic Energy Weapons (KEW) and
Directed Energy Weapons (DEW) have been proposed and en-gineering development has begun.

Because they are not currently deployed, KEW and DEW
systems will be only briefly discussed with the focus on
potential energy weapons - chemical and nuclear explosives.

12.2 KINETIC ENERGY WEAPONS (KEW)

Like all SDI weapons systems, KEW are designed to intercept
ICBMs/SLBMs in various stages of flight-boost, post boost,
mid-course, and terminal. The non-nuclear kinetic kill ve-hicle (KKV) has three kill levels, delineated in Table 12-1.
The KKV's, known as "smart rocks" or "brilliant pebbles" are
designed to impart their tremendous kinetic energy (1/2 mV2
where V is on the order of 5-10 Km/sec) to a target, result-ing in an immediate or delayed (aerothermal structural- ATS)
kill. Four major KEW programs have evolved: SBI, ERIS,HEDI
and hypervelocity electromagnetic launchers.

12.2.1 Space Based Interceptor (SBI)

The SBI system consists of rocket propelled KKV's launched
from orbiting space stations at targets in the boost and
post boost phases. KKV size must be minimized to increase
velocity but this significantly increases the accuracy and
terminal guidance difficulty. An ingenious solution is to
have the KKV "bloom" just prior to impact, spreading its
destruction over a wider area of the target.

12.2.2 Exoatmospheric Reentry Vehicle Interceptor Subsystem

(ERIS)

ERIS is a ground-based rocket system designed to intercept
its target in the mid-course phase, well above the atmo-sphere. Like the SBI, its payload must meet stringent
weight and cost requirements. Because mid-course is the
longest flight phase, ERIS has the most time to accomplish
its mission.

12.2.3 High Endoatmospheric Defense Interceptor (HEDI)

HEDI is designed for atmospheric intercepts at very high velocity with a non-nuclear kill. To accomplish this, HEDI
uses a high-explosive fragmentation warhead detonated very
close to the target, resulting in aerothermal structural
kill.

12.2.4 Hypervelocity Electromagnetic Launcher (EML)

Although originally conceived as space-based, the EML is now
envisioned as a terminal defense system in the low endoat-mospheric region. EML uses electromagnetic forces rather
than rockets to propel a KKV down a barrel at more than 10
KM/S. But high-current armature arcing, mechanical erosion
of the bore, and near-melting point temperatures with rapid
fire operations make employment of this launcher very doubt-ful in the near term.

12.3 DIRECTED ENERGY WEAPONS (DEW)

Directed energy weapons deposit their highly concentrated
energy levels on the surface and interior of their targets.
Lasers kill by burning through the target's skin or impart-ing such a high impulse on the skin that it spalls, destroy-ing vital interior systems or resulting in aerothermal
structural kill. Neutral particle beams penetrate the skin
ionizing as it transits. Inside the target, its damage is
done by ionization of materials in its path. Besides poss-ibly ionizing electronics (resulting in a soft kill), the
energy deposited in the high explosives surrounding the
nuclear warheads may be sufficient to ignite them, giving a
non-nuclear hard kill. DEW programs have evolved in three
areas: the space based chemical laser, the free electron
laser, and neutral particle beam.

12.3.1 Space Based Chemical Laser (SBCL)

The advantage of being space based gives the quick reaction
laser the opportunity to destroy ICBM's in their most vul-nerable stages. A hydrogen-flouride (HF) chemical laser is
designed to destroy targets in the boost and post-boost
phases. Although the technology for this system is mature
(begun in the '70's), the large number of space platforms
and the limited fuel supply carried on each mitigate against
its deployment unless transportation can be made less
expensive.

12.3.2 Ground Based Free Electron Laser (GBFEL)

Through space based relay and fighting mirrors, this high
energy laser is designed to direct its energy at ballistic
missiles in the boost and post-boost phases. Several ground
based stations would provide the lasers. The free electron
laser is among the newest SDI technologies with inherent
problems. Besides the power inefficiency associated with all lasers, the laser's transmission through the atmosphere
will present heretofore insoluble problems.

12.3.3 Neutral Particle Beam (NPB)

This space based weapon system has the potential for both
target kill and discrimination in the boost, post-boost, and
midcourse stages. Despite 50 years of accelerator experien-ce, present technology cannot meet the requirements for low
mass, and continuous, high power levels.

12.4 POTENTIAL ENERGY WEAPONS

An explosion is a change in the state of matter that results
in rapid and violent release of energy. From this broad
definition, explosions may be divided into three types:
mechanical, chemical, and nuclear. Mechanical explosions,
such as the disruption of a steam boiler, are of little con-cern in weapons applications and are not discussed here. For
our purposes, an explosion must be suitable for military
use. In this context, chemical and nuclear explosions
apply.

An explosive may be defined as a material (chemical or
nuclear) that can be initiated and undergo very rapid, self-propagating decomposition, resulting in:

(1) the formation of more stable material;
(2) the liberation of heat;

(3) the development of a sudden pressure effect
through the action of heat on produced or adjacent
gases.

One of the basic properties by which a weapon's
effectiveness is measured is the quantity of energy, and
thus damage potential, it delivers to the target. Modern
weapons use both kinetic and potential energy systems to
achieve maximum lethality. Kinetic energy systems rely on
the conversion of kinetic energy (1/2 MV2) into work, while
potential energy systems use explosive energy directly in
the form of heat and blast or by accelerating the warhead
case fragments to increase their kinetic energy and damage
volume.

A typical modern projectile might have a mass of 25 kg
and contain 20 kg of explosive in a 5 kg case. If the pro-jectile strikes the target going 450 meters per second, the
kinetic energy delivered would by KE = 1/2 MV2 = 1/2 (25)
(450)2 = 2.53 X 106 joules or about 1.01 X 105 J/kg. If the
chemical explosive were detonated on impact, an additional
60 X 106 joules of energy would be released, or 2.5 X 106
J/kg, a factor of 25 increase. For a totally kinetic energy
system to impart this energy, it would have to be traveling
at approximately 2,237 m/s. These high speeds are difficult to maintain over long ranges, although some armor-piercing
shells approach 2,100 m/s; thus, the use of chemical explo-sives plays a major role in modern warheads.

12.5 CHEMICAL EXPLOSIVE REACTION

A chemical explosive is a compound or mixture which, upon
the application of heat or shock, decomposes or rearranges
with extreme rapidity, yielding much gas and heat. Many
substances not ordinarily classed as explosives may do one,
or even two, of these things. For example, a mixture of
nitrogen and oxygen can be made to react with great rapidity
and yield the gaseous product nitric oxide; yet the mixture
is not an explosive since it does not evolve heat, but
rather absorbs heat.

N2 + O2 --> 2NO - 43,200 calories

For a chemical to be an explosive, it must exhibit all
of the following:

(1) Formation of gases

(2) Evolution of heat

(3) Rapidity of reaction

(4) Initiation of reaction

12.5.1 Formation of Gases.

Gases may be evolved from substances in a variety of ways.
When wood or coal is burned in the atmosphere, the carbon
and hydrogen in the fuel combine with the oxygen in the
atmosphere to form carbon dioxide and steam, together with
flame and smoke. When the wood or coal is pulverized, so
that the total surface in contact with the oxygen is in-
creased, and burned in a furnace or forge where more air can
be supplied, the burning can be made more rapid and the com-bustion more complete. When the wood or coal is immersed in
liquid oxygen or suspended in air in the form of dust, the
burning takes place with explosive violence. In each case,
the same action occurs: a burning combustible forms a gas.

12.5.2 Evolution of Heat.

The generation of heat in large quantities accompanies every
explosive chemical reaction. It is this rapid liberation of
heat that causes the gaseous products of reaction to expand
and generate high pressures. This rapid generation of high
pressures of the released gas constitutes the explosion. It
should be noted that the liberation of heat with insuffic-ient rapidity will not cause an explosion. For example, al-though a pound of coal yields five times as much heat as a
pound of nitroglycerin, the coal cannot be used as an explo-sive because the rate at which it yields this heat is quite
slow.

12.5.3 Rapidity of Reaction.

Rapidity of reaction distinguishes the explosive reaction
from an ordinary combustion reaction by the great speed with
which it takes place. Unless the reaction occurs rapidly,
the thermally expanded gases will be dissipated in the med-ium, and there will be no explosion. Again, consider a wood
or coal fire. As the fire burns, there is the evolution of
heat and the formation of gases, but neither is liberated
rapidly enough to cause an explosion.

12.5.4 Initiation of Reaction.

A reaction must be capable of being initiated by the applic-ation of shock or heat to a small portion of the mass of the
explosive material. A material in which the first three
factors exist cannot be accepted as an explosive unless the
reaction can be made to occur when desired.

12.6 CATEGORIES OF CHEMICAL EXPLOSIVES

Explosives are classified as low or high explosives accord-ing to their rates of decomposition. Low explosives burn
rapidly (or deflagrate). High explosives ordinarily deton-ate. There is no sharp line of demarcation between low and
high explosives. The chemical decomposition of an explosive
may take years, days, hours, or a fraction of a second. The
slower forms of decomposition take place in storage and are
of interest only from a stability standpoint. Of more in-terest are the two rapid forms of decomposition, burning and
detonation. The term "detonation" is used to describe an
explosive phenomenon of almost instantaneous decomposition.
The properties of the explosive indicate the class into
which it falls. In some cases explosives may be made to
fall into either class by the conditions under which they
are initiated. For convenience, low and high explosives may
be differentiated in the following manner.

12.6.1 Low Explosives.

These are normally employed as propellants. They undergo
autocombustion at rates that vary from a few centimeters per
second to approximately 400 meters per second. Included in
this group are smokeless powders, which will be discussed in
a later chapter, and pyrotechnics such as flares and
illumination devices.

12.6.2 High Explosives.

These are normally employed in warheads. They undergo
detonation at rates of 1,000 to 8,500 meters per

second. High explosives are conventionally subdivided into
two classes and differentiated by sensitivity:

12.6.2.1 Primary. These are extremely sensitive to shock,
friction, and heat. They will burn rapidly or detonate if
ignited.

12.6.2.2 Secondary. These are relatively insensitive to
shock, friction, and heat. They may burn when ignited in
small, unconfined quantities; detonation occurs otherwise.

12.7 CHARACTERISTICS OF MILITARY EXPLOSIVES

To determine the suitability of an explosive substance for
military use, its physical properties must first be inves-tigated. The usefulness of a military explosive can only be
appreciated when these properties and the factors affecting
them are fully understood. Many explosives have been stud-ied in past years to determine their suitability for mili-tary use and most have been found wanting. Several of those
found acceptable have displayed certain characteristics that
are considered undesirable and, therefore, limit their use-fulness in military applications. The requirements of a
military explosive are stringent, and very few explosives
display all of the characteristics necessary to make them
acceptable for military standardization. Some of the more
important characteristics are discussed below:

12.7.1 Availability and Cost.

In view of the enormous quantity demands of modern warfare,
explosives must be produced from cheap raw materials that
are nonstrategic and available in great quantity. In addi-tion, manufacturing operations must be reasonably simple,
cheap, and safe.

12.7.2 Sensitivity. Regarding an explosive, this refers to
the ease with which it can be ignited or detonated--i.e.,the
amount and intensity of shock, friction, or heat that is re-
quired. When the term sensitivity is used, care must be ta-ken to clarify what kind of sensitivity is under discussion.
The relative sensitivity of a given explosive to impact may
vary greatly from is sensitivity to friction or heat. Some
of the test methods used to determine sensitivity are as
follows:

(1) Impact--Sensitivity is expressed in terms of the
distance through which a standard weight must be dropped to
cause the material to explode.

(2) Friction--Sensitivity is expressed in terms of
what occurs when a weighted pendulum scrapes across the
material (snaps, crackles, ignites, and/or explodes).

(3) Heat--Sensitivity is expressed in terms of the
temperature at which flashing or explosion of the material
occurs.

Sensitivity is an important consideration in selecting
an explosive for a particular purpose. The explosive in an
armor-piercing projectile must be relatively insensitive, or
the shock of impact would cause it to detonate before it
penetrated to the point desired.

12.7.3 Stability.

Stability is the ability of an explosive to be stored
without deterioration. The following factors affect the
stability of an explosive:

(1) Chemical constitution--The very fact that some
common chemical compounds can undergo explosion when heated
indicates that there is something unstable in their struc-tures. While no precise explanation has been developed for
this, it is generally recognized that certain groups, nitro
dioxide (NO2), nitrate (NO3), and azide (N3), are intrin-sically in a condition of internal strain. Increased strain
through heating can cause a sudden disruption of the mole-cule and consequent explosion. In some cases, this condi-tion of molecular instability is so great that decomposition
takes place at ordinary temperatures.

(2) Temperature of storage--The rate of decomposition
of explosives increases at higher temperatures. All of the
standard military explosives may be considered to be of a
high order of stability at temperatures of -10o to +35oC,
but each has a high temperature at which the rate of decom-position becomes rapidly accelerated and stability is re-duced. As a rule of thumb, most explosives becomes danger-ously unstable at temperatures exceeding 70oC.

(3) Exposure to sun--If exposed to the ultraviolet
rays of the sun, many explosive compounds that contain ni-trogen groups will rapidly decompose, affecting their sta-bility.

12.7.4 Power.

The term power (or more properly, performance) as it is
applied to an explosive refers to its ability to do work.
In practice it is defined as its ability to accomplish what
is intended in the way of energy delivery (i.e., fragments,
air blast, high-velocity jets, underwater bubble energy,
etc.). Explosive power or performance is evaluated by a
tailored series of tests to assess the material for its
intended use. Of the test listed below, cylinder expansion
and air-blast tests are common to most testing programs, and
the others support specific uses.

(1) Cylinder expansion test--A standard amount of
explosive is loaded in a cylinder usually manufactured of
copper. Data is collected concerning the rate of radial
expansion of the cylinder and maximum cylinder wall
velocity. This also establishes the Gurney constant or
2E.

(2) Cylinder fragmentation test--A standard steel
cylinder is charged with explosive and fired in a sawdust
pit. The fragments are collected and the size distribution
analyzed.

(3) Detonation pressure (Chapman-Jouget)--Detonation
pressure data derived from measurements of shock waves
transmitted into water by the detonation of cylindrical
explosive charges of a standard size.

(4) Determination of critical diameter--This test
establishes the minimum physical size a charge of a specific
explosive must be to sustain its own detonation wave. The
procedure involves the detonation of a series of charges of
different diameters until difficulty in detonation wave
propagation is observed.

(5) Infinite diameter detonation velocity--Detonation
velocity is dependent on landing density (c), charge dia-meter, and grain size. The hydrodynamic theory of detona-tion used in predicting explosive phenomena does not include
diameter of the charge, and therefore a detonation velocity,
for an imaginary charge of infinite diameter. This proced-ure requires a series of charges of the same density and
physical structure, but different diameters, to be fired and
the resulting detonation velocities interpolated to predict
the detonation velocity of a charge of infinite diameter.

(6) Pressure versus scaled distance--A charge of spec-ific size is detonated and its pressure effects measured at
a standard distance. The values obtained are compared with
that for TNT.

(7) Impulse versus scaled distance--A charge of spec-ific size is detonated and its impulse (the area under the
pressure-time curve) measured versus distance. The results
are tabulated and expressed in TNT equivalent.

(8) Relative bubble energy (RBE)--A 5- to 50-kg charge
is detonated in water and piezoelectric gauges are used to
measure peak pressure, time constant, impulse, and energy.

The RBE may be defined as

Kx 3

RBE = Ks

where K = bubble expansion period for experimental (x) or
standard (s) charge.

12.7.5 Brisance.

In addition to strength, explosives display a second charac-teristic, which is their shattering effect or brisance (from
the French meaning to "break"), which is distinguished form
their total work capacity. This characteristic is of prac-tical importance in determining the effectiveness of an ex-plosion in fragmenting shells, bomb casings, grenades, and
the like. The rapidity with which an explosive reaches its peak pressure is a measure of its brisance. Brisance values
are primarily employed in France and the Soviet Union.

12.7.6 Density.

Density of loading refers to the unit weight of an explosive
per unit volume. Several methods of loading are available,
and the one used is determined by the characteristics of the
explosive. The methods available include pellet loading,
cast loading, or press loading. Dependent upon the method
employed, an average density of the loaded charge can be ob-tained that is within 80-95% of the theoretical maximum den-sity of the explosive. High load density can reduce sensi-tivity by making the mass more resistant to internal fric-tion. If density is increased to the extent that individual
crystals are crushed, the explosive will become more sensi-tive. Increased load density also permits the use of more
explosive, thereby increasing the strength of the warhead.

12.7.7 Volatility.

Volatility, or the readiness with which a substance vapori-zes, is an undesirable characteristic in military explo-sives. Explosives must be no more than slightly volatile at
the temperature at which they are loaded or at their highest
storage temperature. Excessive volatility often results in
the development of pressure within rounds of ammunition and
separation of mixtures into their constituents. Stability,
as mentioned before, is the ability of an explosive to stand
up under storage conditions without deteriorating. Volatil-ity affects the chemical composition of the explosive such
that a marked reduction in stability may occur, which re-sults in an increase in the danger of handling. Maximum
allowable volatility is 2 ml. of gas evolved in 48 hours.

12.7.8 Hygroscopicity.

The introduction of moisture into an explosive is highly
undesirable since it reduces the sensitivity, strength, and
velocity of detonation of the explosive. Hygroscopicity is
used as a measure of a material's moisture-absorbing tenden-cies. Moisture affects explosives adversely by acting as an
inert material that absorbs heat when vaporized, and by act-ing as a solvent medium that can cause undesired chemical
reactions. Sensitivity, strength, and velocity of detona-tion are reduced by inert materials that reduce the contin-uity of the explosive mass. When the moisture content evap-orates during detonation, cooling occurs, which reduces the
temperature of reaction. Stability is also affected by the
presence of moisture since moisture promotes decomposition
of the explosive and, in addition, causes corrosion of the
explosive's metal container. For all of these reasons, hy-groscopicity must be negligible in military explosives.

12.7.9 Toxicity.

Due to their chemical structure, most explosives are toxic
to some extent. Since the effect of toxicity may vary from
a mild headache to serious damage of internal organs, care
must be taken to limit toxicity in military explosives to a
minimum. Any explosive of high toxicity is unacceptable for
military use.

12.8 MEASUREMENT OF CHEMICAL EXPLOSIVE REACTION

The development of new and improved types of ammunition re-quires a continuous program of research and development. A-doption of an explosive for a particular use is based upon
both proving ground and service tests. Before these tests,
however, preliminary estimates of the characteristics of the
explosive are made. The principles of thermochemistry are
applied for this process.

Thermochemistry is concerned with the changes in inter-nal energy, principally as heat, in chemical reactions. An
explosion consists of a series of reactions, highly exo-thermic, involving decomposition of the ingredients and re-combination to form the products of explosion. Energy
changes in explosive reactions are calculated either from
known chemical laws or by analysis of the products.

For most common reactions, tables based on previous in-vestigations permit rapid calculation of energy changes.
Products of an explosive remaining in a closed calorimetric
bomb (a constant-volume explosion) after cooling the bomb
back to room temperature and pressure are rarely those pre-sent at the instant of maximum temperature and pressure.
Since only the final products may be analyzed conveniently,
indirect or theoretical methods are often used to determine
the maximum temperature and pressure values.

Some of the important characteristics of an explosive
that can be determined by such theoretical computations are:

(1) Oxygen balance

(2) Heat of explosion or reaction

(3) Volume of products of explosion

(4) Potential of the explosive

12.8.1 Oxygen Balance (OB%)

Oxygen balance is an expression that is used to indicate the
degree to which an explosive can be oxidized. If an explo-sive molecule contains just enough oxygen to convert all of
its carbon to carbon dioxide, all of its hydrogen to water,
and all of its metal to metal oxide with no excess, the mol-ecule is said to have a zero oxygen balance. The molecule
is said to have a positive oxygen balance if it contains more oxygen than is needed and a negative oxygen balance if
it contains less oxygen than is needed. The sensitivity,
strength, and brisance of an explosive are all somewhat de-pendent upon oxygen balance and tend to approach their maxi-mums as oxygen balance approaches zero.

The oxygen balance (OB) is calculated from the empiric-al formula of a compound in percentage of oxygen required
for complete conversion of carbon to carbon dioxide, hydrog-en to water, and metal to metal oxide.

The procedure for calculating oxygen balance in terms
of 100 grams of the explosive material is to determine the
number of gram atoms of oxygen that are excess or deficient
for 100 grams of a compound.

- 1600 Y

OB (%) = Mol. Wt. of Compound 2X + 2 + M - Z

where

X = number of atoms of carbon

Y = number of atoms of hydrogen

Z = number of atoms of oxygen

M = number of atoms of metal (metallic oxide produced).

In the case of TNT (C6H2(NO2)3CH3),

Molecular weight = 227.1

X = 7 (number of carbon atoms)

Y = 5 (number of hydrogen atoms)

Z = 6 (number of oxygen atoms)

Therefore

OB (%) = -1600 [14 + 2.5 - 6]

227.1

= - 74% for TNT

Because sensitivity, brisance, and strength are prop-erties resulting from a complex explosive chemical reaction,
a simple relationship such as oxygen balance cannot be de-pended upon to yield universally consistent results. When
using oxygen balance to predict properties of one explosive
relative to another, it is to be expected that one with an
oxygen balance closer to zero will be the more brisant, pow-erful, and sensitive; however, many exceptions to this rule
do exist. More complicated predictive calculations, such as
those discussed in the next section, result in more accurate
predictions.

One area in which oxygen balance can be applied is in
the processing of mixtures of explosives. The family of explosives called amatols are mixtures of ammonium nitrate
and TNT. Ammonium nitrate has an oxygen balance of +20% and
TNT has an oxygen balance of -74%, so it would appear that
the mixture yielding an oxygen balance of zero would also
result in the best explosive properties. In actual practice
a mixture of 80% ammonium nitrate and 20% TNT by weight
yields an oxygen balance of +1%, the best properties of all
mixtures, and an increase in strength of 30% over TNT.

12.8.2 Heat of Explosion.

When a chemical compound is formed from its constituents,
the reaction may either absorb or give off heat. The quan-tity of heat absorbed or given off during transformation is
called the heat of formation. The heats of formations for
solids and gases found in explosive reactions have been de-termined for a temperature of 15oC and atmospheric pressure,
and are normally tabulated in units of kilocalories per gram
molecule. (See table 12-1). Where a negative value is giv-en, it indicates that heat is absorbed during the formation
of the compound from its elements. Such a reaction is call-ed an endothermic reaction. The convention usually employed
in simple thermochemical calculations is arbitrarily to take
heat contents of all elements as zero in their standard
states at all temperatures (standard state being defined as
the state at which the elements are found under natural or
ambient conditions). Since the heat of formation of a
compound is the net difference between the heat content of
the compound and that of its elements, and since the latter
are taken as zero by convention, it follows that the heat
content of a compound is equal to its heat of formation in
such nonrigorous calculations. This leads us to the princi-ple of initial and final state, which may be expressed as
follows: "The net quantity of heat liberated or absorbed in
any chemical modification of a system depends solely upon
the initial and final states of the system, provided the
transformation takes place at constant volume or at constant
pressure. It is completely independent of the intermediate
transformations and of the time required for the reactions."

From this it follows that the heat liberated in any
transformation accomplished through successive reactions is
the algebraic sum of the heats liberated or absorbed in the
different reactions. Consider the formation of the original
explosive from its elements as an intermediate reaction in
the formation of the products of explosion. The net amount
of heat liberated during an explosion is the sum of the
heats of formation of the products of explosion, minus the
heat of formation of the original explosive.

The net heat difference between heats of formations of
the reactants and products in a chemical reaction is termed
the heat of reaction. For oxidation this heat of reaction
may be termed heat of combustion.

Table 12-2. Order of Priorities

Priority

Composition of Explosive Products of Decomposition

____________________________________________________________

1 A metal & chlorine Metallic chloride(solid)

2 Hydrogen & chlorine HCL (gaseous)

3 A metal & oxygen Metallic oxide (solid)

4 Carbon & Oxygen CO (gaseous)

5 Hydrogen & oxygen H2O (gaseous)

6 CO and oxygen CO2 (gaseous)

7 Nitrogen N2 (elemental)

8 Excess oxygen O2 (elemental)

9 Excess hydrogen H2 (elemental)

____________________________________________________________

In explosive technology only materials that are exothermic--that is, have a heat of reaction that causes net liberation
of heat--are of interest. Hence, in this text, heats of re-action are virtually all positive. Since reactions may oc-cur either under conditions of constant pressure or constant
volume, the heat of reaciton can be expressed at constant
pressure or at constant volume. It is this heat of reaction
that may be properly expressed as "heat of the explosion."

12.8.3 Balancing Chemical Explosion Equations.

In order to assist in balancing chemical equations, an order
of priorities is presented in table 12-2. Explosives con-taining C, H, O, and N and /or a metal will form the prod-
ucts of reaction in the priority sequence shown. Some ob-servation you might want to make as you balance an equation:

(1) The progression is from top to bottom; you may
skip steps that are not applicable, but you never back up.

(2) At each separate step there are never more than
two compositions and two products.

(3) At the conclusion of the balancing, elemental
forms, nitrogen, oxygen, and hydrogen, are always found in
diatomic form.

Example

TNT:C6H2(NO2)3CH3; constituents: 7C + 5H + 3N + 6O

Using the order of priorities in table 12-1, priority 4
gives the first reaction products:

7C + 6O > 6CO with one mol of carbon remaining

Next, since all the oxygen has been combined with the carbon
to form CO, priority 7 results in:

3N > 1.5N2

Finally, priority 9 results in: 5H > 2.5H2

The balanced equation, showing the products of reaction
resulting from the detonation of TNT is:

C6H2(NO2)3CH3 > 6CO + 2.5H2 + 1.5N2 + C

Notice that partial mols are permitted in these calcula-tions. The number of mols of gas formed is 10. The prod-uct, carbon, is a solid.

12.5.4 Volume of Products of Explosion.

The law of Avogadro states that equal volumes of all gases
under the same conditions of temperature and pressure con-tain the same number of molecules. From this law, it fol-lows that the molecular volume of one gas is equal to the
molecular volume of any other gas. The molecular volume of
any gas at 0oC and under normal atmospheric pressure is very
nearly 22.4 liters or 22.4 cubic decimeters. Thus, consid-ering the nitroglycerin reaction.

C3H5(NO3)3 > 3CO2 + 2.5H2O + 1.5N2 + .25O2

the explosion of one gram molecule of nitroglycerin produces
in the gaseous state: 3 gram molecules of CO2; 2.5 gram mol-ecules of O2. Since a molecular volume is the volume of one
gram molecule of gas, one gram molecule of nitroglycerin
produces 3 + 2.5 + 1.5 + .25 = 7.25 molecular volumes of
gas; and these molecular volumes at 0oC and atmospheric
pressure form an actual volume of 7.25 X 22.4 = 162.4 liters
of gas. (Note that the products H2O and CO2 are in their
gaseous form.)

Based upon this simple beginning, it can be seen that
the volume of the products of explosion can be predicted for
any quantity of the explosive. Further, by employing Char-les' Law for perfect gases, the volume of the products of
explosion may also be calculated for any given temperature.
This law states that at a constant pressure a perfect gas
expands 1/1273 of its volume at 0oC, for each degree of rise
in temperature.

Therefore, at 15oC the molecular volume of any gas is,

V15 = 22.4 (1 + 15/273) = 23.63 liters per mol

Thus, at 15oC the volume of gas produced by the explosive
decomposition of one gram molecule of nitroglycerin becomes

V = 23.63 l (7.25 mol) = 171.3 liters

mol

12.8.5 Potential and Relative Strength of the Explosive.

The potential of an explosive is the total work that can be
performed by the gas resulting from its explosion, when ex-panded adiabatically from its original volume, until its
pressure is reduced to atmospheric pressure and its temper-ature to 15oC. The potential is therefore the total quanti-ty of heat given off at constant volume when expressed in
equivalent work units and is a measure of the strength of
the explosive.

An explosion may occur under two general conditions:
the first, unconfined, as in the open air where the pressure
(atmospheric) is constant; the second, confined, as in a
closed chamber where the volume is constant. The same a-
amount of heat energy is liberated in each case, but in the
unconfined explosion, a certain amount is used as work en-ergy in pushing back the surrounding air, and therefore is
lost as heat. In a confined explosion, where the explosive
volume is small (such as occurs in the powder chamber of a
firearm), practically all the heat of explosion is conserved
as useful energy. If the quantity of heat liberated at con-stant volume under adiabatic conditions is calculated and
converted from heat units to equivalent work units, the
potential or capacity for work results.

Therefore, if

Qmp represents the total quantity of heat given off by
a gram molecule of explosive of 15oC and constant
pressure (atmospheric);

Qmv represents the total heat given off by a gram mol-ecule of explosive at 15oC and constant volume;and

W represents the work energy expended in pushing back
the surrounding air in an unconfined explosion and
thus is not available as net theoretical heat;

Then, because of the conversion of energy to work in
the constant pressure case,

Qmv = Qmp + W

from which the value of Qmv may be determined. Subsequent-ly, the potential of a gram mol of an explosive may be cal-culated. Using this value, the potential for any other
weight of explosive may be determined by simple proportion.

Using the principle of the initial and final state, and
heat of formation table (resulting from experimental data),
the heat released at constant pressure may be readily
calculated.

m n

Qmp = viQfi - vkQfk

1 1

where:

Qfi = heat of formation of product i at constant
pressure

Qfk = heat of formation of reactant k at constant
pressure

v = number of mols of each product/reactants (m is
the number of products and n the number of reactants)

The work energy expended by the gaseous products of
detonation is expressed by:

Since heats of formation are calculated for standard atmo-spheric pressure (10.132 X 104N/m2) and 15oC, V2 is the
volume occupied by the product gases under these conditions.
At this point

W = 10.132 X 104 N )(23.63 l )(Nmol)

m2 mol

and by applying the appropriate conversion factors, work is
determined in units of kcal/mol.

W = (10.132 X 104 N)(23.63 l )(Nmol)(10-3m3

m2 mol l

Joules 1 Kcal

Newton-meter 4185 Joules

Consolidating terms:

W = (.572)(Nmol) Kcal
mol

Once the chemical reaction has been balanced, one can
calculate the volume of gas produced and the work of
expansion. With this completed, the calculations necessary
to determine potential may be accomplished.

For TNT:

C6H2(NO2)3CH3 > 6CO + 2.5H2 + 1.5N2 + C

with Nm = 10 mols

Then:

Qmp = 6(26.43) (+16.5) = 142.08 Kcal
mol

Note: Elements in their natural state (H2, O2, N2, C, et,.)
are used as the basis for heat of formation tables and are
assigned a value of zero. See table 12-2.

Qmv = 142.08 + .572(10) = 147.8 Kcal
mol

As previously stated, Qmv converted to equivalent work units
is the potential of the explosive. (MW = Molecular Weight
of Explosive)

Potential = Qmv Kcal 4185 J 103g 1mol

mol Kcal Kg MW gm

Potential = Qmv (4.185 x 106) Joules

MW Kg

For TNT,

Potential = 147.8 (4.185 x 106) = 2.72 x 106 J

227.1 Kg

Rather than tabulate such large numbers, in the field
of explosives, TNT is taken as the standard explosive, and
others are assigned strengths relative to that of TNT. The
potential of TNT has been calculated above to be 2.72 X 106
Joules/kg. Relative strength (RS) may be expressed as

R.S. = Potential of Explosive/2.72 X 106

12.8.6 Example of Thermochemical Calculations

The PETN reaction will be examined as an example of thermo-chemical calculations.

Then the remaining oxygen will combine with the CO
to form CO and CO2.

5CO + 3O > 2CO + 3CO2

Finally the remaining nitrogen forms in its natur-al state (N2).

4N > 2N2

The balanced reaction equation is:

C(CH2ONO2)4 > 2CO + 4H2O + 3CO2 + 2N2

(2) Determine the number of molecular volumes of gas
per gram molecule. Since the molecular volume of one gas is
equal to the molecular volume of any other gas, and since
all the products of the PETN reaction are gaseous, the re-sulting number of molecular volumes of gas (Nm) is:

Nm = 2 + 4 + 3 + 2 = 11 mol-volume

mol

(3) Determine the potential (capacity for doing work).
If the total heat liberated by an explosive under constant
volume conditions (Qm) is converted to the equivalent work
units, the result is the potential of that explosive.

The heat liberated at constant volume (Qmv) is
equivalent to the liberated at constant pressure (Qmp) plus
that heat converted to work in expanding the surrounding
medium. Hence, Qmv = Qmp + Work (converted).

a. Qmp = Qfi (products) - Qfk (reactants)

where: Qf = Heat of Formation (see table 12-2)

For the PETN reaction:

Qmp = 2(26.43) + 4(57.81) + 3(94.39) - (119.4)

= 447.87 Kcal

mol

(If the compound produced a metallic oxide, that
heat of formation would be included in Qmp.

b. Work = .572(Nm) = .572(11) = 6.292 Kcal

mol

As previously stated, Qmv converted to equivalent work
units is taken as the potential of the explosive.

c. Potential J = Qmv (4.185 x 106

Kg MW

= 454.16 (4.185 x 106)

316.15

= 6.01 x 106 J

Kg

This product may then be used to find the relative
strength of PETN, which is

e. RS = Pot (PETN = 6.01 x 106 = 2.21

Pot (TNT) 2.72 x 106

12.6 REFERENCES/BIBLIOGRAPHY

Army Research Office. Elements of Armament Engineering
(Part One). Washington, D.C.: U.S. Army Material Command,
1964.